The present invention relates to permanent magnet motors, and more particularly to sensorless rotor position estimators for permanent magnet motors.
Interest in electric and hybrid electric vehicles (EV and HEV) is increasing due to more stringent emissions standards. EV and HEV vehicles require a highly efficient, reliable and safe electric drivetrain to compete with vehicles with internal combustion (IC) engines. Using efficient motor drives and advanced control methods such as sensorless techniques for deriving rotor position reduces the weight and cost of the electric drivetrain and improves the operating efficiency of the EV and HEV vehicles.
Interior permanent magnet (IPM) motor drives have natural saliency, which refers to the variation in stator leakage inductance with respect to rotor position. Saliency-based sensing systems derive the position of the rotor without rotor position transducers, hall effect sensors, or other physical sensors. In other words, the motor acts as an electromagnetic resolver. A power converter applies a carrier frequency voltage to a stator winding of the motor. The stator winding produces high frequency currents that vary with rotor position. The current variations are sensed by a current sensor.
Referring now to FIG. 1, a negative sequence component (NSC) of stator current signal is shown at 10. The NSC of the stator current signal 10 is processed to generate a rotor position signal that is labeled 12 in FIG. 1. The NSC current variations have a relatively small amplitude (e.g., 3 Amps) as compared with fundamental stator currents (e.g., 300 Amps). Transients in the stator current create harmonics over the entire frequency spectrum, including a near negative sequence carrier signal frequency. A Fast Fourier Transform (FFT) 14 of the NSC of stator current illustrates the harmonic content. The harmonics prevent accurate measurement of the negative sequence carrier signal current. In other words, conventional sensorless rotor position estimators tend to temporarily generate inaccurate rotor position estimates. Because the near negative sequence carrier signal frequency contains the desired saliency spatial information, it is difficult or impossible to accurately determine the position of the rotor.
As was previously mentioned, the NSC of the stator current is very small as compared with the fundamental stator current. It is very difficult to accurately measure the small amplitude signals using a sensor that is rated for much higher current levels. Injecting higher amplitude currents onto the stator coils would possibly improve accuracy. Using a current sensor with lower current ratings would also improve accuracy. Both options are not viable for drive applications. Injecting higher current onto the stator coil increases losses in the drive system. Using a current sensor with a lower current rating as compared with the rated current limits the torque generating capability of the drive system.
A rotor position estimator according to the present invention estimates rotor position for a permanent magnet motor that includes a stator and a rotor. A sensing circuit generates d-axis and q-axis negative sequence stationary current (NSSC) signals. A signal conditioning circuit combines the d-axis and q-axis NSSC signals with a first positive feedback signal that is based on a rotor position estimate signal to generate modified d-axis and q-axis NSSC signals. A regulator is coupled to an output of the signal conditioning circuit. A mechanical system simulator is coupled to an output of the regulator and generates the rotor position estimate signal.
In other features of the invention, the signal conditioning circuit combines the modified d-axis and q-axis NSSC signals with a second positive feedback signal that is based on a rotor position estimate signal. The mechanical system simulator receives a demand torque signal.
In still other features, the signal conditioning circuit includes a first multiplier having first inputs that receive the d-axis and q-axis NSSC signals. The signal conditioning circuit includes a second harmonic amplifying circuit having an input that receives the rotor position estimate signal and an output that produces the first feedback signal to a second input of the first multiplier. The first multiplier multiples the first feedback signal and the d-axis NSSC signal to generate the modified d-axis NSSC signal. The first multiplier also multiples the first feedback signal and the q-axis NSSC signal to generate the modified q-axis NSSC signal.
In yet other features, the signal conditioning circuit includes a second multiplier having first inputs that receive the modified d-axis and q-axis NSSC signals from the first multiplier and an output that is coupled to the regulator. The signal conditioning circuit includes an inverse saliency model that has an input that receives the rotor position estimate signal and that generates the second feedback signal that is output to a second input of the second multiplier. The regulator is preferably selected from the group of proportional (P), proportional integral (PI), proportional integral differential (PID), and limited PI regulators.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.